1. Field of the Invention
The present invention relates to a color conversion film in the form of a solid-state, thin film that has fluorescence or phosphorescence, that can be processed by a coating process, and that is formed using a conjugated high molecular weight copolymer having high color conversion efficiency. Moreover, the present invention relates to a multicolor light-emitting organic EL device that is formed to include the color conversion film.
2. Background of the Related Art
Considerable research has been conducted on practical application of organic EL elements in recent years. Organic EL elements are expected to realize high photoluminescent brightness and photoluminescent efficiency since they are capable of realizing high current density at low voltages. Particularly high expectations are being placed on practical application of high-definition organic multicolor EL displays capable of multicolor or full-color display. An example of a method for realizing multicolor or full-color organic EL displays consists of using a plurality of types of color filters that allow transmission of light of a specific wavelength regime (color filter method). In the case of applying the color filter method, the organic EL element used is required to emit so-called “white light” comprising a proper balance of light of the three primary colors (red (R), green (G) and blue (B)) as multicolor light.
A method for obtaining multicolor light-emitting organic EL element is proposed in Japanese Patent Application Laid-open No. 2000-243563 (Patent Document 1) in which a light-emitting layer containing a plurality of luminescent pigments is used to simultaneously excite the plurality of luminescent pigments. In addition, U.S. Pat. No. 5,683,823 (Patent Document 2) proposes a method for causing energy transfer to a guest material and light emission simultaneous to causing excitation and emission of light from a host light-emitting material by using a light-emitting layer that contains a host light-emitting material and a guest light-emitting material.
However, the above-mentioned multicolor light-emitting organic EL elements depend on either simultaneous excitation of a plurality of types of light-emitting materials or energy transfer between a plurality of types of light-emitting materials. In such devices, the balance of emission intensity between the light-emitting materials has been reported to change as drive time elapses or accompanying changes in applied current, thereby resulting in the risk of a change in the resulting color hue.
Japanese Patent Application Laid-open No. 2002-75643 (Patent Document 3) and Japanese Patent Application Laid-open No. 2003-217859 (Patent Document 4) propose a color conversion method that uses a monochromatic light-emitting organic EL element and a color conversion film as another method for obtaining a multicolor light-emitting organic EL element. The color conversion film is a layer containing one or a plurality of color conversion substances that absorb light of a short wavelength and convert it to light of a long wavelength.
However, when a color conversion film is formed with a single color conversion substance, a phenomenon referred to as concentration quenching occurs. Concentration quenching refers to dissipation (quenching) of light without being emitted while energy absorbed by a color conversion substance repeatedly moves between molecules. In order to suppress this phenomenon, Japanese Patent Application Laid-open No. 2000-230172 (Patent Document 5) proposes a medium dispersion type of color conversion film that lowers the concentration of a color conversion substance in a color conversion film by dissolving or dispersing the color conversion substance in some type of medium. This medium dispersion type of color conversion film is able to inhibit contact between color conversion substances accompanying a decrease in concentration of the color conversion substance.
In this case, if the concentration of the color conversion substance is lowered, the absorbance of light to be absorbed decreases, preventing the obtaining of sufficient converted light intensity. In response to this problem, the thickness of the color conversion film is increased to increase absorbance and maintain color conversion efficiency. In the case of using a thick color conversion film (having a film thickness of about 10 μm) in this manner, problems occur such as a large level difference, difficulty in achieving high definition and residual moisture or solvent in the film. The presence of a large level difference results in the occurrence of wire breakage in the electrode patterns at the locations of level differences in the case of attempting to form an organic EL element on the color conversion film. In addition, the presence of residual moisture or solvent in the color conversion film causes deterioration of the organic EL layer by the residual moisture or solvent when combined with an organic EL element, thereby resulting in display defects.
Moreover, in a medium dispersion type of color conversion film as described above, the color conversion substance frequently diffuses into the medium and gathers to form aggregates with the passage of time, thereby causing concentration quenching. Thus, the use of this type of color conversion film makes it difficult to maintain high color conversion efficiency over a long period of time.
Therefore, in order to realize a color conversion film that is capable of maintaining sufficient converted light intensity over a long period of time without increasing thickness, a material is desired that has suitable absorption and emission spectra, has high fluorescent quantum yield, and does not cause concentration quenching when thickness is reduced. In the past, few materials were found that were able to efficiently absorb ultraviolet light or blue light and convert it to green, light, and there are particularly few materials useful for multicolor light-emitting organic EL devices using a color conversion film that are capable of efficiently absorbing blue light and converting it to green light. Thus, there was a strong desire to find such a color conversion film or material for realizing a multicolor light-emitting organic EL device using that color conversion film.
Moreover, in order to produce such a color conversion film at low cost, it is desirable to use a wet process such as a coating process that uses a solution of a color conversion material after dissolving the color conversion material in a suitable solvent. However, the color conversion substance is required to be soluble in order to achieve this. Moreover, from the viewpoint of the process, it is desirable that viscosity be able to be easily adjusted to a viscosity that is suitable for the apparatus used. In order to achieve this object, it is desirable to use a color conversion substance composed of a polymer material that offers a wide selection range of applicable solvents and enables viscosity to be easily adjusted by changing the molecular weight thereof.
Conventional examples of color conversion films using a polymer material include a color conversion film containing a polymer material having a polyarylenevinylene group as a repeating unit thereof proposed in Japanese Patent Application Laid-open No. 2000-26852 (Patent Document 6), and a color conversion film containing a polymer material having fluorene as a repeating unit thereof proposed in Japanese Patent Application Laid-open No. 2006-169265 (Patent Document 7). In addition, as described in Appl. Phys. Lett., 61, 2793 (1992) (Non-Patent Document 1) and Nature, 365, 628 (1992) (Non-Patent Document 2), polyarylenevinylene derivatives have been widely researched as candidates for light-emitting materials of polymer EL elements due to their comparatively high fluorescent quantum yield and high solubility. Color conversion films using polymer materials are also required to have high fluorescent quantum yield. However, the fluorescent quantum yields of polyarylenevinylene derivatives in the form of thin films that have been reported thus far have been on the order of 10 to 40%, thus demonstrating values that are inadequate for use as color conversion films.
An object of the present invention is to provide a green light-emitting color conversion film capable of maintaining sufficient converted light intensity over a long period of time without increasing thickness, and being able to be produced by a low-cost, wet process. In addition, another object of the present invention is to provide a multicolor light-emitting organic EL device using the color conversion film that has superior viewing angle dependency and demonstrates stable emission properties over a long period of time without a change in hue with the passage of drive time or accompanying a change in applied current.